Chapter 06 · The Science

Quanta & the Great Debate

Here is the great irony of Einstein's life. He was one of the founders of quantum theory — it won him his Nobel Prize — yet he could never accept the strange, probabilistic world it revealed. For thirty years he argued with it, and lost most of the arguments. But his objections were so deep that they are still shaping physics today.

The man who made light grainy

In his 1905 paper on the photoelectric effect, Einstein proposed that light is not a smooth wave but a stream of discrete energy packets — quanta, later called photons. Each photon's energy depends only on the light's frequency (its colour), not its brightness: E = hf.

This explained a puzzle no wave theory could: why dim blue light can knock electrons out of a metal while bright red light cannot. It was, in his own words, "very revolutionary" — and it earned him the 1921 Nobel Prize.

Sources:
  1. Wikipedia: Photoelectric effect.
  2. Wikipedia: Photon.

Try it yourself

The photoelectric effect

Shine light on a metal. Below a certain frequency, nothing happens, no matter how bright. Above it, electrons fly off instantly. Slide the colour from red toward ultraviolet and find the threshold.

Your prediction

The light is already above the threshold and electrons are flying out. Now you make it bluer – a higher frequency – but with the same number of light quanta per second. What changes for the electrons?

Interactive · quantum physics

Light vs. electrons

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Kinetic energy of ejected electrons: 0 eV

The threshold here is set for a sodium-like metal (work function ≈ 2.3 eV). Notice that brightness never appears in the calculation — only frequency decides whether electrons escape. That is the quantum surprise.

Answer

BEach electron flies out with more energy.

Light comes in portions, the photons, and the energy of a photon depends only on the frequency. Bluer light has more energetic photons; whatever is left over after freeing the electron, it carries away as energy of motion. How many electrons come out depends instead on the number of photons per second (for a given colour: on the brightness). Below the threshold, brighter light does not help. This is how Einstein explained the photoelectric effect in 1905; chiefly for this he received the 1921 Nobel Prize in Physics.

For the curious: the calculation

Energy of motion E = h · f − W with h ≈ 4.14 · 10⁻¹⁵ eV·s and the work function W = 2.3 eV. At 6 · 10¹⁴ Hz: 2.48 eV − 2.3 eV ≈ 0.18 eV. At 10 · 10¹⁴ Hz: 4.14 eV − 2.3 eV ≈ 1.84 eV.

Source: Wikipedia: Photoelectric effect

Challenges0 of 2 done

  • Find the threshold: the lowest frequency at which electrons only just come out.
  • Set a frequency at which the electrons fly out with at least 2 eV.
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Sources:
  1. Wikipedia: Photoelectric effect — work function and threshold frequency.

More quantum gifts

Einstein's quantum contributions went far beyond 1905.

Lasers

In 1916 he described stimulated emission — one photon triggering another identical one. Decades later this became the principle behind every laser.

Bose–Einstein condensate

With Satyendra Nath Bose he predicted a strange new state of matter near absolute zero. It was finally created in a lab in 1995, winning its makers a Nobel Prize.

Wave–particle duality

His work helped reveal that light, and matter, behave as both wave and particle — a cornerstone of modern physics.

Sources:
  1. Stimulated emission; Bose–Einstein condensate.

"God does not play dice"

Niels Bohr and Albert Einstein in deep discussion, photographed by Paul Ehrenfest around 1925
Einstein and Niels Bohr, around 1925. Their good-humoured but profound debates over quantum theory are legendary in physics.Image: Paul Ehrenfest. Public domain, via Wikimedia Commons (Source)

By the late 1920s, quantum mechanics had become deeply probabilistic: it could predict the odds of where an electron would be found, but never a definite answer. To Einstein this was intolerable. He believed a complete theory must describe a definite reality, not just probabilities.

At the famous Solvay Conferences he sparred with Niels Bohr, inventing ingenious thought experiments to trip up quantum theory. Bohr, sometimes after a sleepless night, found an answer to each of them. "God does not play dice," Einstein insisted; in a letter to Born in 1926 he wrote: "I, at any rate, am convinced that He does not throw dice." In Bohr's own later account, he cautioned against ascribing attributes to Providence in everyday language. The quip "Einstein, stop telling God what to do" is a later retelling, not a documented quote.

The 1927 Solvay Conference group photograph, with seventeen of the twenty-nine attendees being or becoming Nobel laureates
The 1927 Solvay Conference in Brussels — arguably the most extraordinary gathering of minds in the history of science. Einstein sits at the centre of the front row.Image: Benjamin Couprie, Institut International de Physique Solvay. Public domain, via Wikimedia Commons (Source)
Quantum mechanics is certainly imposing. But an inner voice tells me that it is not yet the real thing.Albert Einsteinletter to Max Born, 4 December 1926, English translation
Sources:
  1. Wikipedia: Bohr–Einstein debates.
  2. Niels Bohr: Discussion with Einstein on Epistemological Problems in Atomic Physics (1949); Wikiquote: Albert Einstein (letter to Born, 4 December 1926).
  3. Wikipedia: EPR paradox; quantum entanglement.